Waterborne coating composition

ABSTRACT

The invention relates to a method of making an aqueous coating composition comprising a blend of at least a first aqueous polymer dispersion and a second aqueous polymer dispersion comprising a film-forming second polymer wherein the first aqueous polymer dispersion comprises a first polymer having a number average molecular weight, Mn, of from 2000 to 120000, an acid value of 30 to 150 mg KOH/g, a glass transition temperature Tg of at least 20° C. as calculated with the Fox formula, and an ethylene-oxide content of from 1 to 20 wt %, the method comprising blending an aqueous dispersion of the first polymer having a solids content of 25 to 50 wt % and a pH of 4.5 to 8.0 with an aqueous dispersion of the second polymer having a solids content of 25 to 55 wt % and a pH of 4.5 to 8.0.

BACKGROUND OF THE INVENTION

The invention relates generally to a polymer; a method of making thepolymer; an aqueous dispersion comprising the polymer as a first polymerand a second dispersed polymer different from the first polymer; amethod of making the aqueous dispersion; an aqueous coating compositioncomprising said aqueous dispersion; the use of the first polymer as anopen time and wet edge time extender; a method of making said coatingcomposition; a method of applying the coating composition to a substrateand an article coated with a cured layer of the coating composition.

DESCRIPTION OF THE RELATED ART

Legislation on the emission of volatile compounds is driving the switchfrom solvent borne to waterborne coating compositions. However,waterborne paints still need improvement to achieve, or to match, theadvantageous properties of their solvent borne predecessors. Aparticular problem with waterborne coating compositions is the shortperiod of time in which a freshly applied layer of coating can bere-manipulated without leaving brush marks, roller marks, or visiblelines at joins between adjacent coating layers.

When a coating composition has been freshly applied to a substrate andis still in a state where it can be manipulated during its curing periodwithout marking it is said to be open. The open time is the period oftime between the point of first application of a coating layer up to thepoint where corrections can no longer be made in the wet paint filmwithout leaving visible marks, such as brush marks or spray dust(commonly known as ‘overspray’).

When a coating composition has been first applied to a substrate and itis still possible to blend it with a subsequently added adjacent layerof coating during its curing period without a visible seam the coatingis said to have a wet edge. The wet edge time is the time period betweenthe point of fresh application of a coating layer up to the point whenit is no longer fluid enough to blend with adjacent areas in a mannerthat does not leave a visible seam or lapline.

It is desirable to improve these characteristics for waterborne paints.

Examples of aqueous coating compositions that may benefit from anincreased open time and wet edge time include: aqueous vinyl polymerdispersions, aqueous alkyd emulsions, aqueous polyurethane dispersionsand mixes thereof. Such polymer dispersions can have excellent dryingproperties, chemical and water resistance and favorable mechanicalproperties such as hardness, block and scratch resistance but coatingcompositions based on these binders alone typically suffer from a shortopen or wet edge time.

It is believed to be the case that longer open time and wet edge timecan be achieved by using water-soluble additives or co-binders in theformulation of coatings. For example, water-soluble co-solvents such asalkylene glycols (e.g. ethylene glycol, propylene glycol) are consideredto have a beneficial influence on open time. Alternatively,water-soluble homopolymers are suggested to be useful to achieveextended open times in waterborne coatings (Examples of such polymersbeing polyvinyl alcohol, polyethylene glycol, poly(meth) acrylamide,polyvinylpyrrolidone or poly 2-ethyl oxazoline). Other alternativesinclude water-dispersible film-forming polymers, such as are describedin U.S. Pat. No. 4,552,908, which have been suggested to be useful informulating paints with improved brushability, lapping, and flow-out ofbrush-marks. Film-forming polymers in aqueous coating compositionstypically are meant to be polymers that have a glass transition belowroom temperature, typically below 60, 45 or more commonly below 30° C.,optionally in combination with a glass transition modifying substance,such that they have sufficient flowability to form a coherent film on asubstrate in ambient use temperatures, preferably in a temperature rangebetween 0 and 60° C.

However, it is a problem with water-soluble and water-dispersiblepolymers or oligomers that, because the content of hydrophilic groups inthe polymers or oligomers is high, the resulting coatings exhibit poorwater-resistance. This implicates only low levels can be applied, thusreducing the effectiveness of improving open time or wet edge time.

EP 0136025 describes the use of a polymer of defined molecular weightwhich contains either anionisable or cationisable groups and alsonon-ionisable water-soluble moieties (derived for example frompolyethylene glycol) for formulating paints with extended open time.Since this polymer is completely water-soluble and is used as the solebinder the coating described exhibit poor water resistance.

U.S. Pat. No. 4,139,514 discusses addition of a water-soluble oligomerto a coating. The description refers to addition of an acid richoligomer to latex to achieve an open time in excess of 20 minutes.However, the technology is limited to using an alkali-soluble oligomerwhich will result in water sensitivity of the final coating, thusrequiring the addition of a melamine crosslinker and curing at elevatedtemperatures in order to achieve water resistance.

EP1132436 discusses the addition of polyalkylene oxide group modified(meth)acrylic monomer units to aqueous high gloss acrylic polymerdispersions. The polymer described however is thermoplastic so thechemical resistance and mechanical properties are expected to beinferior.

US 20010031826 A1 discusses addition of polyalkylene oxide groupmodified (meth)acrylic monomer units to aqueous high gloss acrylicpolymer emulsions used as single binder in glossy paint formulations toimprove open time. In order to obtain acceptable coatings properties ina thermoplastic polymer the molecular weight has to be high, typicallyabove Mn 200,000. Consequently a high concentration of ethylene oxidemonomer is necessary to obtain the desired application properties. Thehigh concentration, however, will negatively influence the waterresistance.

EP1492850 discusses blending of a non-crosslinking polyethylene glycolmodified vinyl oligomer with a dispersed polymer. This documentdiscusses inclusion of vinyl polymers incorporating 0-45 wt %polyethylene oxide functionality into oligomers used to extend open timeand wet edge time. It is believed that claimed single use of non-ionichydrophilic functionality—when used in a water dispersed polymer—leadsto problems like water sensitivity, coagulation, depletion flocculationand other stability problems. When using the necessary high amount ofnon-ionic hydrophilic groups to obtain the claimed open time/wet edgeproperties, leads to unacceptable water resistance and stability.

EP 210747 discusses inclusion of eicosanols as evaporation suppressantsin coating compositions to increase open time. However, becauseeicosanol additive boiling points are so high as to prevent theirevaporation during curing, the additive remains in the cured coating andthus leads to water sensitivity problems.

Despite the progress made in the art, there remains a need for furtherimprovement of the open time and wet edge time properties of waterbornecoating compositions.

THE INVENTION

Therefore, according to the invention there is provided an aqueouscoating composition comprising a blend of at least a first aqueouspolymer dispersion and a second aqueous polymer dispersion comprising afilm-forming second polymer wherein the first aqueous polymer dispersioncomprises a first polymer having a number average molecular weight, Mn,of from 2000 to 120000 (determined by gel permeation chromatographyusing a mixture of tetrahydrofurane and acetic acid as eluent), an acidvalue of 30 to 150 mg KOH/g, and a ethylene-oxide content of from 1 to20 wt %, and is obtainable by free radical polymerization of a monomermixture comprising:

a) 5 to 20 wt % acid functional ethylenically unsaturated monomers orprecursors thereof or ethylenically unsaturated monomers comprisingionic group precursors

b) 5 to 25 wt % ethylenically unsaturated monomers containing apolyethylene glycol or mono alkoxy polyethylene glycol moiety;

c) up to 90 wt % of non-ionic ethylenically unsaturated monomers otherthan a or b;

d) 0 to 10 wt % ethylenically unsaturated monomers with a functionalgroup for crosslinking

e) 0 to 10 wt % of chain transfer agents;

wherein the sum of a) to e) is 100 wt %.

The first polymer is part of the coating composition and has thefunction of improving the open time and/or wet edge time of the filmforming second polymer dispersion in the coating composition. Withoutany limitation to theory, it is believed that the particular monomermakeup of the first polymer provides for a polymer that is able to delaythe onset of the phase transition from oil in water emulsion to water inoil emulsion that takes place on drying of a coating, and that evenafter this phase transition, a reverse transition is possible uponaddition of fresh coating composition or water to the applied layer.

The first polymer and second polymer are present in the coatingcomposition as two distinct blended dispersion systems, that is, theparticles of the first polymer dispersion are distinct from the secondpolymer dispersion. It is believed that by mechanical blending of twodifferent, stable aqueous dispersions of polymer particles, the polymerparticles of each separate dispersion remain discrete and distinct fromone another without substantial mixing of the polymers trapped withineach particle. Preferably the aqueous coating composition comprises ablend of at least a first polymer binder existing as discrete particleand an second aqueous polymer binder also present as discrete particles.

Examples of suitable aqueous second dispersions are those wherein filmforming second polymer is a vinyl polymer, polyurethane or alkyd orcombinations thereof wherein the ratio—calculated on solid polymers—ofthe first to the second polymer preferably is 10/90 to 70/30, preferably40/60 to 60/40. In a preferred embodiment the second polymer can be avinyl polymer bearing carbonyl groups for cross-linking with a carbonylreactive cross-linker. The second polymer can be an auto-oxidisablecrosslinking organic polymer containing unsaturated fatty acid residues,preferably an auto-oxidisable polyurethane. In a particular embodimentthe second polymer is an alkyd emulsion or urethane modified alkydemulsion.

DETAILED DESCRIPTION OF THE INVENTION

In a particular aspect the invention relates to an aqueous dispersion,for use in the coating composition according to the invention as thefirst aqueous polymer dispersion, comprising a first polymer having anumber average molecular weight (Mn) of from 2,000 to 120,000(determined by gel permeation chromatography using a mixture oftetrahydrofurane and acetic acid as eluent), an acid value of from 30 to150 mg KOH/g, and an ethylene-oxide wt % (on total solid polymer) offrom 1 to 20 wt %, said first polymer dispersion being obtainable byfree radical polymerization of a monomer mixture in the presence of atleast one free-radical initiator and at least one surfactant, saidmonomer mixture comprising:

a) 5 to 20 wt %, preferably 7 to 10 wt %, acid functional ethylenicallyunsaturated monomers or precursors thereof or ethylenically unsaturatedmonomers comprising ionic group precursors;

b) 5 to 25 wt %, preferably 7 to 20 wt %, ethylenically unsaturatedmonomers containing polyethylene oxide, polyethylene glycol ormono-alkoxypolyethylene glycol moeity

c) up to 90 wt % of non-ionic ethylenically unsaturated monomers otherthan a) or b);

d) 0 to 10 wt % ethylenically unsaturated monomers with a functionalgroup for crosslinking

e) 0 to 10 wt % of chain transfer agents;

f) up to 90 wt % non-ionic ethylenically unsaturated monomers other thanc), wherein 30 to 90 wt %, more preferably 60 to 80 wt % comprisecrosslinkable groups or precursors thereof;

wherein the sum of a) through f) is 100 wt %.

The surfactant preferably is is a reactive surfactant comprising a freeradically reactive double bond, preferably having a general structureM+.—OOC—CH═CHCOOR, R—O—(CH₂—CH₂—O)_(n)—R₁ or RO—(CH₂—CH₂—O)n-X or blendsthereof, wherein R₁ is an alkyl or hydrogen group, wherein X is ananionic group, preferably a sulphate or phosphate salt and wherein R isan organic group comprising: an allylic or maleic free radicallyreactive double bond and an alkyl, aryl or aralkyl group containing atleast 8 carbons; and n is an integer from 0 to 50, more preferably 2 to25, even more preferably 2 to 10, and most preferably 3 to 8 and M+ is acation, preferably Na+, K+, Li+, NH4+ or a protonated or quaternaryamine.

acid functional ethylenically unsaturated monomers

or precursors thereof or ethylenically unsaturated monomers comprisingionic group

In the aqueous dispersion the acid functional ethylenically unsaturatedmonomers or precursors thereof or ethylenically unsaturated monomerscomprising ionic group precursors a) comprise, respectively, carboxylicacid groups or carboxylic acid group precursors or sulphonic andphosphonic acid groups; preferred monomers are selected from the groupconsisting of: methacrylic acid, acrylic acid, itaconic acid, maleicacid or anhydride, citraconic acid and fumaric acid.

The polyethylene oxide containing ethylenically unsaturated monomers b)preferably comprise monomers of the general formulaR₃—O—(CH₂—CH₂—O)_(n)—R₂, wherein R₃ is acryl or methacryl; R₂ is H oralkyl, preferably comprising from 1 to 4 carbon atoms, n is an integerfrom 1 to 35, preferably more preferably 2 to 20 and most preferably 3to 15.

The non-ionic ethylenically unsaturated monomers c) preferably comprise:

i. up to 100 wt % styrene and/or styrene derivatives

ii. up to 100 wt % of one or more non-ionic(meth)acrylic monomers

iii. up to 25 wt % vinyl monomers containing oxidative cross linkinggroups;

iv. up to 10 wt % vinyl monomers containing self cross linkable groups,preferably ketone functional monomers;

v. up to 15 wt % vinyl monomers containing hydroxyl groups;

vi. up to 5 wt % vinyl monomers containing wet adhesion promoters

wherein the sum of the weight of monomers i-vi=100 wt % of non-ionicethylenically unsaturated monomers ci)

The first polymer can have a number average molecular weight (Mn)ranging from 2,000 to 120,000, but preferably from 4,000 to 12,000 andmost preferably 5,000 to 10,000. The weight average molecular weight(Mw) is preferably from 8,000 to 50,000 and most preferably 10,000 to25,000. The first polymer has a preferred polydispersity—as defined bythe ratio of Mn/Mw—of 1.2 to 3.0, preferably 1.5 to 2.5. The numberaverage molecular weight as well as the weight average molecular weightcan be determined by gel permeation chromatography using THF/acetic acidas eluent. Methods to influence the molecular weight in emulsionpolymerization are well known to those skilled in the art and aredescribed for example in “Emulsion Polymerization, A MechanisticApproach” by Bob Gilbert, 245-291, Academic Press, 1995.

Further, the Tg of the first polymer is preferably in the range 0 to 80°C., preferably 45 to 70° C., most preferably 50 to 70° C. The firstpolymer preferably has an acid value 45-80 mg KOH, wherein preferably inthe aqueous composition the acid functional groups of the polymer areneutralized to an α value of from 0.05 to 0.70, preferably 0.10 to 0.25and the aqueous dispersion has a pH of from 6.0 to 8.0, most preferably6.5 to 7.5. Further, the polymer preferably has a an ethyleneoxide wt %of 1 to 20 wt %, preferably 5 to 12 wt %, on total solids.

The first polymer is obtainable by free-radical polymerization,preferably by aqueous emulsion polymerization. A general description ofthe emulsion polymerization process is given in E. W. Duck, Encyclopediaof Polymer Science and Technology (John Wiley & Sons, Inc.: 1966), Vol.5, pp. 801-859. In a particular embodiment the first polymer may have agradient morphology. In the process surfactants are used. Surfactantsperform many functions in emulsion polymerization, includingsolubilizing hydrophobic monomers, determining the number and size ofthe dispersion particles formed, providing dispersion stability asparticles grow, and providing dispersion stability duringpost-polymerization processing. Typical examples of surfactants used inemulsion polymerization are anionic surfactants like fatty acid soaps,alkyl carboxylates, alkyl sulphates, and alkyl sulfonates; nonionicsurfactants like ethoxylated alkylphenol or fatty acids used to improvefreeze thaw and shear stability. Often a combination of anionicsurfactants or anionic and nonionic surfactants is used to provideimproved stability.

A preferred alternative for using conventional surfactants is the use ofreactive surfactants. These preferred surfactants have an ethylenicallyunsaturated bond that can participate in a free radical polymerization.Preferably the ethylenically unsaturated bond is a maleic or allylicdouble bond. These double bonds have a low reactivity compared to the(meth)acrylic or vinyl double bonds in the monomers a) to d) whichensures that the monomers a) to d) are preferentially reacted in thepolymerisation of the first polymer over the reaction with thesurfactant. Reversely, it is also preferred that the monomers a) to d)comprise (meth)acrylic and/or vinyl double bonds and no or nosubstantial amount of maleic or allylic double bonds. The reactivesurfactants further have a hydrophobic moeity comprising an alkyl, arylor aralkyl group containing at least 6, preferably at least 8 and morepreferably at least 10 carbons and a hydrophilic moeity formed by anionic group and/or a polyethyleneoxide group comprising between 0 and50, preferably 2-25 even more preferably between 2 and 10 and mostpreferably 3-8 ethyleneoxide units. The reactive surfactants arepreferably of a low molecular weight molecules and typically have amolecular weight of less than about 3000, less than 2500, or less than2000 gr/mol. The reactive surfactants can be used either assole-emulsifier or in combination with conventional surfactants.Suitable polymerizable surfactants include hemi-esters of maleicanhydride of the formula

M+.—OOC—CH═CHCOOR wherein R is C(6-22) alkyl and M+ is Na+, K+, Li+,NH4+, or a protonated or quaternary amine. Other suitable polymerizablesurfactants include polyoxyethylene alkylphenyl ethers with anethylenically unsaturated bond represented by the following structure:

where R is alkyl with at least 8 carbon atoms, n is 2-25 and X is H,—SO₃H or —P(═O)(OH)₂ or salts thereof. Commercially available fromMontello, Inc., sold under the tradename Noigen® RN MAXEMUL™ 6106(available from Croda Industrial Specialties), which has bothphosphonate ester and ethoxy hydrophilicity, a nominal C18 alkyl chainwith reactive group that can react with (meth)acrylic monomers. Otherrepresentative reactive surfactants with phosphate ester functionalitiessuitable for such reactions include, but are not limited to, MAXEMUL™6112, MAXEMUL™ 5011, MAXEMUL™ 5010 (all available from Croda IndustrialSpecialties). Alternative reactive surfactants suitable for use withvarious embodiments of the present invention include sodium allyloxyhydroxypropyl sulphonate (available from Rhodia as SIPOMER COPS-1™),surfactants having the following formula:

where R is alkyl with at least 8 carbon atoms, n is 2-25 and X is H,—SO₃H or —P(═O)(OH)₂ or salts thereof. Commercially available from AdekaCo., Ltd. Under the tradename ADEKA REASOAP SR/ER series such as ADEKAREASOAP ER-10, ER-20, ER-30 and ER-40, Akeda Reasope SR-10, SR-20, SR-30and allylsulphosuccinate derivatives (such as TREM LF-40™ (availablefrom Cognis)).

In a preferred embodiment a chain transfer agent is used to bring themolecular weight between 2000 and 120000. Exemplary chain transferagents are butyl mercaptan, mercaptopropionic acid, 2-ethylhexylmercaptopropionate, n-dodecylmercaptan, t-dodecylmercaptan, n-butylmercaptopropionate, mercaptoethanol, octyl mercaptan, isodecylmercaptan, octadecyl mercaptan, mercaptoacetic acid, allylmercaptopropionate, allyl mercaptoacetate, crotyl mercaptopropionate,crotyl mercaptoacetate. Other, non sulfur based chain transfer agentinclude halogenated hydrocarbons or catalytic chain transfer agents suchas Cobalt-chelates such as used in N. S. Enikolopyan et al, J. Polym.Chem. Ed, Vol 19, 879 (1981). Also alpha-methyl styrene dimer oroligomers of alpha-methyl styrene dimer can be used as explained in US2007/0043156 A1 and U.S. Pat. No. 6,872,789. Yet another method tosynthesize polymer with a well defined molecular weight is the use ofdiarylethene. The use of diarylethene is described in detail in W.Bremser et al, Prog. Org. Coatings, 45, (2002), 95 and JP 3135151, DE10029802 and US 2002/0013414. A commonly used diarylethene includesdiphenylethene.

The first polymer comprises 5 to 20 wt %, preferably 7 to 10 wt % ofacid functional ethylenically unsaturated monomers or ethylenicallyunsaturated monomers comprising acid functional group precursors. Theseacid functional groups can be carboxylic, sulphonic or phosphonic.Carboxylic acid groups are particularly advantageous because they lendthemselves well to full dissociation to the ionic form when neutralized,so that they aid solubility in water, but have poor water solubilitywhen not neutralized after evaporation of the neutralizing agent.Preferably, monomers such as (meth)acrylic acid are used. Other possiblecarboxylic acid-functional monomers are itaconic acid, fumaric acidmaleic acid, citraconic acid, or the anhydrides thereof. Besidesmonomers having carboxylic acid functionality also monomers possessingan acid-functional group other than the carboxylic one can be present inthe monomer composition, such as ethylmethacrylate-2-sulfonic acid or2-acrylamido-2-methylpropane sulfonic acid. The concentration of theacid functional ethylenically unsaturated monomers is chosen to providethe first polymer with an acid value between 30-150 mg KOH/g, preferably45-80 mg KOH/g on solid polymer as determined by ISO 3682.

The first polymer contains 5 to 25 wt %, preferably 7 to 20 wt % andmost preferably 9-15 wt % polyethylene oxide ethylenically unsaturatedmonomers. The polyethylene oxide ethylenically unsaturated monomerspreferably comprise from 2 to 50 ethylene oxide units, more preferablyfrom 2 to 20, and most preferably from 2 to 15. The monomers arepreferably polyethylene glycol (meth)acrylate monomers or mono methoxy(meth)acrylate monomers. Suitable examples of polyethylene oxideethylenically unsaturated monomers ii) are methoxy polyethylene glycol550 methacrylate and methoxy polyethylene glycol 350 methacrylate.Examples are Visiomer ETMA, Visiomer MPEG550MA (available from Evonik),Bisomer S20W, Bisomer PEA6 (available from Cognis). The concentration ofpolyethylene oxide ethylenically unsaturated monomers is chosen toprovide the first polymer with an ethylene oxide wt % of 1 to 20 wt %,preferably 5 to 12 wt % on solid polymer. The percentage ethylene oxidecalculation is based on the molecular weight of the EO unit—being 44—asa % of the molecular weight of the involved monomer and then taken as a% of the involved monomer over the total solid monomer composition.

As discussed above, the first polymer comprises up to 90 wt % non-ionicethylenically unsaturated monomers c) (other than monomers a) or b)).Preferred monomers include esters of acrylic and methacrylic acid suchas n-butyl (meth) acrylate, methyl (meth) acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth) acrylate, lauryl(meth)acrylate, cycloalkyl(meth) acrylates, e.g., isobornyl (meth) acrylate and cyclohexyl (meth)acrylate; and ethylenically unsaturated compound such as substituted orunsubstituted monovinylidene aromatic monomers including styrene,alpha-methyl styrene, t-butylstyrene, vinyl toluene, o-, m-, andp-methylstyrene, o-, m-, and p-ethylstyrene; dienes such as1,3-butadiene or isoprene; or mixtures thereof. Also, vinyl esters, suchas vinyl acetate, vinyl alkanoate or their derivatives or mixturesthereof can be used in the monomer composition. Nitriles, such as (meth)acrylonitrile can also be used.

Other monomers possessing functional groups other than acidic groups canalso be present in the monomer composition. In some cases thesefunctional groups can be used for crosslinking of the first polymer upondrying. The first polymer can be adapted to crosslink with itself and/orthe second polymer. Examples of monomers d) having crosslinkable groupsinclude hydroxy-functional monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate,also monomers having latent hydroxy groups such as glycidyl methacrylatecan be used. Hydroxy-functional groups can be cross-linked withpolyisocyanates which may be blocked or not, melamines, and urea resins.Further examples include derivatives of (meth) acrylamide such asN-methylol (meth) acrylamide and diacetone acrylamide. Vinyl monomerspossessing an acetoacetoxy functional group can also be present in themonomer composition. Examples of such vinyl monomers areacetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate,acetoacetoxy(methyl)ethyl acrylate, acetoacetoxypropyl acrylate, allylacetoacetate, acetoacetamidoethyl (meth)acrylate, and acetoacetoxybutylacrylate.

The monomers d) possessing functional groups can be used forcrosslinking of the polymer. Crosslinkers for use in combination withthey above functional groups are known to those skilled in the art.Crosslinking can occur at ambient or elevated temperature. Thecrosslinker may be already incorporated in the first polymer dispersionresulting in a self crosslinking polymer or can be added at a laterstage.

A special case of a self crosslinking polymer dispersion is were thefirst polymer has auto oxidative groups like unsaturated fatty acidmodified monomers or (meth)acryloyl modified monomers. This can becatalyzed by metal catalysts. The crosslinking can be catalysed by metalcatalysts such as iron or cobalt complexes. Naturally the second bindercan contain similar groups resulting in co-crosslinking between thebinders.

Preferably the first polymer as discussed above has a glass transitiontemperature Tg of at least 0° C., preferably at least 20° C., morepreferably at least 35° C., and most preferably at least 45° C.Preferably the polymer has a Tg between 0 to 80° C., and more preferablybetween 45 to 70° C. The Tg can be calculated with the Fox formula (seeUllmanns Enzyklopadie der Technischen Chemie, 4^(th) Edition, Vol 19,Weinheim 1980 p 17 and 18) and by using Tg values of the homo-polymersas mentioned in table 1. Alternatively the Tg of the first polymer canbe measured using Differential Scanning calorimetry.

TABLE 1 Tg values (K) of homo-polymers. methyl methacrylate 105 butylmethacrylate 20 methacrylic acid 228 lauryl methacrylate −65 Methoxypolyethylene glycol 230* methacrylate (ETMA) −31 Methoxy polyethyleneglycol 350* methacrylate (MPEG350MA) −17 Methoxy polyethylene glycol550* methacrylate (MPEG550MA) −31 Methoxy polyethylene glycol 750*methacrylate (MPEG750MA) −58 Adduct of sunflower fatty acid and glycidylmethacrylate −60 *The glycol number refers to the average molecularweight of the polyglycol

The first polymer can be used to increase an open time, a wet edge time,and/or the hardness of an aqueous coating composition of a (second)polymer binder. Therefore, the invention also relates to the use of thefirst polymer or of the aqueous dispersion comprising the first polymerto increase an open time, a wet edge time, and/or the hardness of acoating composition.

The second polymer is at least a part of the binder system of thecoating composition. The second polymer can be any aqueous polymerdispersion suitable for use as a waterborne coating composition binder,and is therein preferably cross linkable, and more preferably self crosslinking. The composition of the coating composition can vary in broadranges for example comprising:

-   -   a) 2 to 75 wt %, preferably 30 to 60 wt %, more preferably 45-55        wt % (based on solids content) of the first polymer; and    -   b) 2 to 75 wt %, preferably 30 to 60 wt %, more preferably 45-55        wt % (based on solids content) of the second polymer;    -   based on total weight of a) and b).

The second polymer preferably has a measured weight average molecularweight of from 20,000 to 2,000,000, preferably 30,000 to 1,000,000, mostpreferably 40,000 to 120,000 g/mol. The second polymer preferably has aglass transition temperature Tg of from −30 to 80° C., preferably 0 to45° C., more preferably 0 to 25° C.

Particular examples of such second polymer dispersions are self crosslinking acrylic dispersions such as described in EP 0927198 and EP1125949. Commercially available self-cross linking acrylic dispersionsare Setaqua 6776, Setaqua 6782 available from Nuplex resins or NeoCrylXK-98 available from DSM NeoResins+

Examples of polyurethane or a polyurethane-acrylic hybrid dispersionsinclude the binders described in WO2010/066902. A polyurethanedispersion that is particularly suited to use in the present inventionis fatty acid modified to provide an auto-oxidisable cross linkingpolymer and is described in WO2007/131959. An example of such a polymeris Setaqua X11712, commercially available from Nuplex Resins BV. Anexample of a urethane-acrylic hybrid polymer dispersion is Rhodasol F115also available from Nuplex Resins.

Suitable alkyd emulsions are generally prepared by preparing an alkydbinder by conventional polycondensation methods and emulsifying saidbinder in water afterwards. The hydrophilic groups needed to stabilizethe alkyd particles in the aqueous phase can be ionic or non-ionic andcan be introduced by the use of conventional surfactants or by modifyingthe alkyd during or after the synthesis with stabilizing groups. Anexample of such a polymer is Uradil AZ 554 Z-50, an alkyd dispersion exDSM NeoResins+ or Dynotal LS82 ex Dyno ASA. Optionally, the alkydemulsions are modified with di- or polyisocyanates prior or afteremulsification. Alkyd emulsions thus modified have the advantage ofdrying faster than non-isocyanate modified alkyd emulsions. Examples ofsuch products are Worléesol 150 E from Worlée, Setaqua 6002 and Setal6004 ex Nuplex Resins. Blends of the polymer dispersions mentioned abovecan be used as well to make up the coating composition of the invention.

The polymer dispersions mentioned above have excellent dryingproperties, chemical and water resistance and favorable mechanicalproperties such as hardness, block and scratch resistance but coatingcompositions based on these binders alone typically suffer from a shortopen or wet edge time.

In order to improve the water compatibility of the first polymer and thesecond polymer and increase aqueous dispersion/emulsion stability, thefree acid groups (e.g. carboxylic acid groups) of the acid monomers canbe fully or partly neutralized. The neutralization is performed byaddition of a neutralizing agent. The neutralizing agent can be added tothe monomers or to the produced polymer. Suitable neutralization agentsinclude metallic bases (e.g. potassium and sodium bases) or organicbases such as amines or ammonia, particularly ammonia or2-amino-2-methyl-1-propanol).

The degree of neutralization of the first polymer can be indicated as α,wherein α=0.00 refers to the fully protonic state (no neutralization)and α=1.00 refers to full ionization (fully neutralized). Preferably adispersion or emulsion of the polymer is neutralized to an α value from0.05 to 0.70, more preferably 0.10 to 0.25. The α indicated is based onthe calculated (by molar equivalence) degree of neutralization.

The invention also relates to a method of making a coating compositioncomprising blending an aqueous dispersion of the first polymer having asolids content of 25 to 50 wt %, preferably 30 to 40 wt % and a pH of4.5 to 8.0, preferably 6.5 to 8.0, most preferably 6.5 to 7.5 with anaqueous dispersion of the second polymer having a solids content of 25to 55 wt %, preferably 30 to 50 wt %; and a pH of 4.5 to 8.0, preferably6.5 to 8.0.

Prior to the blending the aqueous dispersions of the first and secondpolymers preferably have difference in pH units less than 2.0, morepreferably less than 1.5, and most preferably less than 1.0.

Advantageously, colloidal stability of the dispersion composition of thefirst and second polymer dispersions can be enhanced by matching the pHof the separate first and second polymer dispersions. This is done byneutralization of the acid functional groups on at least the firstpolymer using a neutralizing agent. The water preferably comprises lessthan 10 gr/ltr calcium ions. The second polymer may also contain ionicgroups, particularly acid groups. To aid dispersibility these groups arealso preferably partly or fully neutralized by addition of neutralizingagent(s).

In neutralizing the first and second polymer dispersions, care must betaken that the respective polymers retain their particulate nature. Thedegree of neutralization of the ionic (preferably acid) functionalgroups is chosen to avoid dissolution of the polymers.

The coating composition preferably comprises of a blend wherein theratio—calculated on solid polymers—of the first to the second polymer is10/90 to 70/30, preferably 40/60 to 60/40. The coating compositionpreferably comprises between 10-70 wt %, preferably 30-50 wt % of thefirst based on total solids of the coating composition.

As with coating compositions generally, auxiliary components can beincluded. In this respect the waterborne coating composition may furthercomprise one or more organic solvents that aid film-formation, a pigment(organic or inorganic) and/or other additives and fillers known in theart. When an organic solvent is used, water miscible solvents arepreferred. The amount of organic solvent shall be chosen in such a wayto provide a coating composition with a low volatile organic content(VOC), and preferably comprises less than 50 g/litre (including water),preferably less than 30 g/litre (including water) of volatile organiccompounds, as calculated by ISO method 11890-2 it the ready to use form.

The coating composition may also include other auxiliary components suchas additives or fillers used in formulating coatings. Examples include,but are not limited to, leveling, rheology, anti-block, and flow controlagents such as silicones, fluorocarbons, urethanes, or cellulosics;extenders; flatting agents; pigment wetting and dispersing agents andsurfactants; ultraviolet (UV) absorbers; UV light stabilizers; tintingpigments; extenders; defoaming and antifoaming agents; anti-settling,anti-sag and bodying agents; anti-skinning agents; anti-flooding andanti-floating agents; fungicides and mildewcides; corrosion inhibitors;thickening agents; plasticizers; reactive plasticizers; drying agents;catalysts; or coalescing agents. The invention also relates to a paintcomposition comprising the coating composition and 1-50 wt % pigment.

The waterborne coating composition is preferably a colloidal dispersionor an emulsion. It preferably has a solids content of between 20 and 70wt %, preferably between 30 and 55 wt % and a water content of 10 to 45wt % (based on total weight of composition), a pH of from 2.0 to 9.0,preferably 4.5 to 8.0, most preferably 7.5 to 8.0.

In the coating composition the first polymer may have functional groupssuch as (meth)acryloyl functionality or oxidative functionality oracetoacetoxy functionality, that can co-cross link with oxidative oracetoacetoxy functionality when present on the second polymer.Alternatively, the first polymer has self cross linkable functionalityand can co-cross link with a selfcross linking functionality on thesecond polymer. In a preferred embodiment at least one of the first orsecond polymer has oxidative cross linking functionality, and preferablythe composition comprises a dryer catalyst, for example a metalcatalysts, preferably cobalt, manganese or iron complexes.

The coating composition according to the invention preferably comprisesno more than 20 wt % volatile organic solvent, preferably 5% and mostpreferably 3%.

The coating composition according to the invention may further comprise:

-   -   a) up to 20 wt % of isocyanate cross linker;    -   b) up to 20 wt % of polyhydrazide cross linker;    -   c) up to 10 wt % of a silane cross linker;    -   d) up to 10% of an (meth)acryloyl oligomer; and    -   e) up to 50 wt % of an aqueous polymeric dispersion or        emulsified resin different to the first and second polymers.

As mentioned earlier the coating compositions in accordance with thepresent invention are suitable for a variety of coatings uses, forexample, as paint, impregnating, sealing and bonding compositions. Apreferred application is as a primer, topcoat, or clearcoat. The coatingcompositions may be applied to a substrate in any convenient manner suchas, for example, by brushing, spraying or dipping. Suitable substratesinclude metals, wood, board, plastics and leather. The first polymeraccording to the invention provides i.a. good open time whereas articlescoated with the coating or paint composition after curing have excellentcoating properties.

The following is a description of certain examples of the invention,given by way of example only.

EXAMPLES

The following is a description of certain examples of the invention,given by way of example only. Examples 1 to 10 were all made asdescribed in below for first polymer example 1. Differences in monomercomposition and neutralization are depicted in tables 2 and 3.

Example 1. Preparation of Aqueous Polymeric Vinyl Dispersion PolymerHaving Both Ethylene Oxide and Carboxylic Acid Functionality (ResinExample 1)

In an emulsion polymerization reactor the pre-emulsion was made asfollows: 454 grams of water and 5 grams of an anionic polymerizablesurfactant (Adeka Reasoap SR 1025, ex Adeka) was added and heated to 70°C. In the feed-tank a mixture of 117 grams of water, 22.8 grams of theanionic polymerizable surfactant Adeka Reasoap SR 1025 and 0.30 grams ofsodium lauryl sulphate were mixed for 5 minutes. The monomerpre-emulsion was prepared by adding the following raw materials in thefeed tank: 109.5 grams of methyl methacrylate, 24.2 grams of methoxypolyethyleenglycol 550 methacrylate (ex. Cognis), 24.2 grams ofmethacrylic acid, 145.7 grams of butyl methacrylate, 3.56 octylmercaptane and 1.75 grams of mercapto ethanol. Emulsify the feed until astable pre-emulsion was obtained.

Add 5 weight % of the pre-emulsion to the reactor. Heat the reactor to80° C. Add a solution of 6.3 grams of water and 0.30 grams of ammoniumpersulphate to the reactor and wait for 5 minutes. Heat the reactor to85° C., start feeding the pre-emulsified monomers from the feedtank andseparately the initiator solution of 15.2 grams of water and 0.75 gramsof ammonium persulphate. The reaction temperature was 85±2° C. Themonomer feed dosing takes 60 minutes. The initiator feed was be 70minutes. The following finishing steps were performed: rinse the feedtank with 21.0 grams of water and the initiator tank with 6.7 grams ofwater. Maintain the temperature for another 60 minutes after the feedhas been completed. Cool the batch to 65° C., add a slurry of 4.0 gramsof water and 0.75 grams of butyl hydroperoxide (70 weight % solution inwater to the reactor and dose the solution 16.0 gram of water and 0.35gram sodium formaldehyde sulphoxylate over a 15 minutes period. Keep thetemperature for another 30 minutes. Cool to 25° C. At 25° C. add asolution of 3.1 gram Proxel AQ in 4.0 gram water, rinse with 4.0 gramsof water and add the solution of 1.65 gram of 25 weight % ammoniasolution in 4.0 grams of water. Rinse with 4.0 grams of water. Anaqueous polymeric vinyl dispersion polymer having both ethylene oxideand carboxylic acid functionality was obtained with the followingspecifications: Solids content=32% pH=7.5; grit <100 ppm; residualmonomer <100 ppm. Particle Size=122 nm. The molecular weight wasdetermined by gel permeation chromatography using THF with 2% aceticacid as eluent: M_(n)=5,000 and M_(w)=13,200.

Examples 2-8. Preparation of Aqueous Polymeric Vinyl Dispersions HavingBoth Ethylene Oxide and Carboxylic Acid Functionality

Further examples were made using the method described in example 1. Themonomer mixtures used to prepare the pre-emulsions is given in table 2.

TABLE 2 Monomer compositions for pre-emulsions. Example 2 3 4 5 6 7Methyl methacrylate 10.96 10.99 10.99 10.99 11.00 10.98 Methoxypolyethylene 4.92 2.34 2.25 2.64 2.43 2.43 glycol methacrylate 550Methacrylic acid 2.42 2.43 2.43 2.43 2.43 2.43 Butyl methacrylate 12.0914.72 14.81 14.41 14.63 14.61 N-Octyl mercaptane 0.36 0.37 0.37 0.370.37 0.37 2-Mercapto ethanol 0.18 0.18 0.18 0.18 0.18The molecular weights determined by GPC are:

Example M_(n) M_(w) 2 7000 14300 3 7300 15400 4 6800 12900 5 6300 132006 9100 19700 7 6500 13700

Examples 8-10. Preparation of Aqueous Polymeric Vinyl Dispersions HavingBoth Ethylene Oxide and Carboxylic Acid Functionality

Further examples were made using the method described in example 1. Themonomer mixtures used to prepare the pre-emulsions is given in table 3.

TABLE 3 Monomer compositions for pre-emulsions. Example 8 9 10 Methylmethacrylate 10.99 10.99 10.01 Methoxy polyethyleenglycol 2.43 0.61 2.21methacrylate 550 Methacrylic acid 2.43 2.43 2.21 Butyl methacrylate 8.4114.63 13.33 Adduct of sunflower fatty acid and 6.20 glycidylmethacrylate Lauryl methacrylate 1.82 N-Octyl mercaptane 0.37 0.37 0.332-Mercapto ethanol 0.18 0.18 0.16For the final neutralization examples 9 and 10 used 0.17 grams ofammonia (aqueous 25%), whereas example 11 used 2.35 grams of2-amino-2-methyl-1-propanol, The molecular weights determined by GPCare:

Example M_(n) M_(w) 8 7200 16300 9 6400 13300 10 6500 17600

Example 10. Coating Formulations According to the Invention

Different coating compositions were prepared according to the invention.All compositions are given in table 4. The oxidatively dryingpolyurethane dispersion used in these coating composition examples wasprepared according to EP 2024412, example 1.

The open of the paint was determined by applying it onto a Leneta plainchart FORM WH with a bar applicator at 125 microns. Using the rubbereraser of a pencil with a width of at least of 2 mm, X-shaped crossesare being applied into the paint layer immediately after if has beenapplied. A brush (Elma acryl 93-14 or Pro-Gold Exclusive 7200-12) loadedwith fresh paint and the excess paint is removed by scraping alongsidethe edge of the can. The fresh loaded brush is moved twice in thevertical direction of the width of the substrate and twice in thehorizontal direction of the length of the substrate at the location ofthe X-shaped cross. This movement is repeated 10 times on the same cross(“10 cross-brushes”). This procedure is repeated after a one ortwo-minute interval on the next cross, until the cross stays visibleeven after 10 “cross-brushes” (one movement=twice in the direction ofthe width of the substrate and twice in the direction of the length ofthe substrate). The open is reported as the time in which the X-crossshape damage in the fresh applied film can be completely removed after10 “cross-brushes” within the next interval of 1 or 2 minutes. 2 minutesintervals may be chosen to get a first indication of the open time; moreaccurate open time measure is followed by taking 1 minutes intervals.

Other properties of the coating compositions: ICI viscosity was measuredaccording to ASTM D 4287, Gloss was measured according to ASTM D 2457and König hardness was measured according to DIN 53157. The data aresummarised in Table 4 which clearly illustrates the improvement in opentime.

TABLE 4 Coating compositions according to the invention. Variation onRatio on solids Koenig ratio of first second ICI Open Gloss 20° hardnessfirst and polymer polymer viscosity time after 7 after 7 second binderdispersion dispersion (Poise) (min) days (%) days (s) First polymer 0100 3.0 10.0 55 dispersion: example 25 75 2.7 9.0 49 1. Second polymer50 50 2.6 12.0 57 dispersion: ex. 1 60 40 2.4 9.0 47 from EP2024412First polymer 0 100 3.4 9.5 74 29 dispersion: example 10 90 3.4 10.0 7233 2. Second polymer 20 80 3.1 10.0 71 34 dispersion: ex. 1 30 70 3.39.5 70 31 from EP2024412 40 60 3.0 11.5 66 35 45 55 3.2 12.0 61 34 50 503.1 14.0 52 35 60 40 2.7 12.5 39 40

Example 11. Coating Compositions

In table 5, coating compositions are given of commercially availableaqueous polymer dispersions (second polymer dispersion) with the aqueouspolymer dispersion from example 2.

TABLE 5 Coating compositions. Koenig Hardness after Gloss after 7 days20° (%) 7 days 50% RH (s) Open time (min) 1 to 1 1 to 1 1 to 1 blendwith blend with blend with dispersion dispersion dispersion from fromfrom Resin As such example 2 As such example 2 As such example 2 Setaqua6004* 54 69 22 35 12.0 17.0 Rhodasol F115* 42 31 108 122 4.0 10.0NeoCryl XK-98** 21 27 45 68 11.0 16.0 Worleesol 150 E*** 60 57 41 6713.0 16.0 *Available from Nuplex Resins **Available from DSM Neoresins+***Available from Worlée

Thus, the invention has been described by reference to certainembodiments discussed above. It will be recognized that theseembodiments are susceptible to various modifications and alternativeforms well known to those of skill in the art.

Further modifications in addition to those described above may be madeto the structures and techniques described herein without departing fromthe spirit and scope of the invention. Accordingly, although specificembodiments have been described, these are examples only and are notlimiting upon the scope of the invention.

What is claimed is:
 1. A method of making an aqueous coating compositioncomprising a blend of at least a first aqueous polymer dispersion and asecond aqueous polymer dispersion comprising a film-forming secondpolymer wherein the first aqueous polymer dispersion comprises a firstpolymer having a number average molecular weight; Mn, of from 2000 to120000 determined by gel permeation chromatography using a mixture oftetrahydrofuran and acetic acid as eluent, an acid value of 30 to 150 mgKOH/g, a glass transition temperature Tg of at least 20° C. ascalculated with the Fox formula, and a ethylene-oxide content of from 1to 20 wt %, and is obtained by free radical polymerization of a monomermixture comprising: a) 5 to 20 wt % acid functional ethylenicallyunsaturated monomers or precursors thereof or ethylenically unsaturatedmonomers comprising ionic group precursors; b) 5 to 25 wt %ethylenically unsaturated monomers containing a polyethylene glycol ormono alkoxy polyethylene glycol moiety; c) up to 90 wt % of non-ionicethylenically unsaturated monomers other than a) or h); d) 0 to 10 wt %ethylenically unsaturated monomers with a functional group forcrosslinking; and e) 0 to 10 wt % of chain transfer agents; wherein thesum of a) to e) is 100 wt %, the method comprising blending an aqueousdispersion of the first polymer having a solids content of 25 to 50 wt %and a pH of 4.5 to 8.0 with an aqueous dispersion of the second polymerhaving a solids content of 25 to 55 wt % and a pH of 4.5 to 8.0.
 2. Themethod of claim 1 wherein the film forming second polymer in the secondaqueous polymer dispersion is a vinyl polymer, polyurethane or alkyd orcombinations thereof.
 3. The method of claim 1 wherein the secondpolymer is an aqueous vinyl polymer dispersion bearing carbonyl groupsfor cross-linking with a carbonyl reactive cross-linker or anauto-oxidisable organic polymer containing unsaturated fatty acidresidues.
 4. The method of claim 1, wherein the second polymer is anauto-oxidisable polyurethane containing unsaturated fatty acid residuesor an alkyd- or urethane modified alkyd emulsion.
 5. The method of claim1 wherein the weight based on the solids content of the first polymer tothe second polymer in the aqueous coating composition is 10/90 to 70/30.6. The method of claim 1 wherein the aqueous coating composition has apH from 4.5 to 8.0.
 7. The method of claim 1 wherein the second polymerhas a weight average molecular weight, Mw, of from 20,000 to 2,000,000g/mol as measured by gel permeation chromatography using a mixture oftetrahydrofuran and acetic acid.
 8. The method of claim 1 wherein thesecond polymer has a glass transition temperature Tg of from −30° C. to80° C.
 9. The method of claim 1 wherein the first polymer and secondpolymer have (meth)acryloyl, oxidative or acetoacetoxy functional groupsfor co-crosslinking.
 10. The method of claim 1 wherein the aqueouscoating composition comprises less than 20 wt % volatile organicsolvent.
 11. The method of claim 1 further comprising addition of one ormore components chosen from the group consisting oil up to 20 wt % ofisocyanate cross linker; up to 20 wt % of polyhydrazide cross linker; upto 10 wt % of a silane cross linker; up to 10 wt % of an (meth)acryloyloligomer; and up to 50 wt % of an aqueous polymer dispersion differentfrom the first and second polymer dispersion.
 12. The method accordingto claim 1 wherein the first polymer dispersion is obtained by freeradical polymerization of the monomer mixture in the presence of atleast one free-radical initiator and at least one surfactant, whereinthe surfactant is a reactive surfactant comprising a free radicallyreactive double bond.
 13. The method according to claim 1 wherein thefirst polymer dispersion is obtained by free radical polymerization ofthe monomer mixture in the presence of at least one free-radicalinitiator and at least one surfactant; wherein the surfactant is areactive surfactant comprising a free radically reactive double bondhaving a general structure M+.—OOC—CH═CHCOOR, RO—(CH2-CH2-O)n-R1 orRO—(CH2-CH2-O)n-X or blends thereof, wherein R1 is an alkyl or hydrogengroup, wherein X is an anionic group, and wherein R is an organic groupcomprising: an allylic or maleic free radically reactive double bond andan alkyl, aryl or aralkyl group containing at least 8 carbons; and n isan integer from 0 to 50 and M+ is Na+, K+, Li+, NH4+ or a protonated orquaternary amine.
 14. The method according to claim 1 wherein themonomer mixture comprises ionic ethylenically unsaturated monomers orprecursors a) comprising one or more groups chosen from the groupconsisting of carboxylic acid groups, carboxylic acid precursor groups,sulphonic acid groups and phosphonic acid groups.
 15. The methodaccording to claim 1 wherein the monomer mixture comprises ethylenicallyunsaturated monomers containing polyethylene oxide b) of the generalformula R₃—O—(CH2-CH2-O)n-R2, wherein R3 is acryl or methacryl; R2 is Hor alkyl, n is an integer from 1 to
 35. 16. The method according toclaim 1 wherein the monomer mixture comprises one or more non-ionicethylenically unsaturated monomers c) chosen from the group consistingof: i. up to 100 wt % styrene and/or styrene derivatives; ii. up to 100wt % of one or more non-ionic(meth)acrylic monomers; iii. up to 25 wt %vinyl monomers containing oxidative crosslinking groups; iv. up to 10 wt% vinyl monomers containing self-crosslinkable groups; v. up to 15 wt %vinyl monomers containing hydroxyl groups and; vi. up to 5 wt % vinylmonomers containing wet adhesion promoters wherein the sum of the weightof monomers i-vi=100 wt % of non-ionic ethylenically unsaturatedmonomers c).
 17. The method according to claim 1 wherein the firstpolymer has a number average molecular weight (Mn) of 4,000 to 12,000g/mol and has a polydispersity of 1.2 to
 3. 18. The method according toclaim 1 wherein the acid functional groups of the first polymer areneutralized to an alpha value of from 0.05 to 0.70 and wherein the pH isfrom 6.0 to 8.0.
 19. The method according to claim 1 wherein the acidfunctional groups of the first polymer are neutralized to an alpha valueof from 0.10 to 0.25 and wherein the pH is from 6.5 to 7.5.
 20. Themethod according to claim 1 wherein the pH difference of the first andsecond polymer dispersion is less than
 2. 21. The method according toclaim 16 wherein the monomer mixture comprises iv) up to 10 wt % vinylmonomers containing self-crosslinkable groups, which are ketonefunctional monomers.
 22. The method according to claim 17 wherein thefirst polymer has a number average molecular weight (Mn) of 5,000 to10,000 g/mol and has a polydispersity of 1.5 to 2.5.